Renewable energy devices such as wind turbine generators (WTGs) and tidal stream generators are increasingly important sources of power for AC electricity networks. Such devices traditionally employ a transmission in the form of a gearbox to change the slow input speed of an energy extraction mechanism such as the rotor of a wind or tidal turbine into a fast output speed to drive a generator. Such gearboxes are challenging to design and build as they are prone to failure and expensive to maintain and replace or repair.
A further challenge in designing renewable energy devices is extracting the optimum amount of energy by the energy extraction mechanism in all conditions. The most effective devices achieve this by holding the blades at a fixed pitch angle, and varying the rotational speed of the blades proportionally to the wind or water speed over the majority of the operating range, so as to maintain a more or less constant ‘tip speed ratio’. Gearboxes at the scale required for cost effective renewable energy devices are invariably fixed ratio, so complex and failure-prone electronic power conversion is required to provide electricity to an AC electricity network.
It has therefore been proposed to build transmissions for renewable energy devices using fluid working machines, it being possible to make such hydrostatic transmissions variable ratio even at large scales. Such a hydrostatic transmission is also lighter and more robust than a gearbox, and lighter than a direct generator drive, which would otherwise perform the same function, and thereby reduces the overall cost of producing electricity. U.S. Pat. No. 4,503,673 (Schacle) disclosed a plurality of variable displacement motors driven by a plurality of pumps, the motor displacement being varied in aggregate to control the hydraulic pressure according to a pressure vs. rotor speed function. WO 2007 053036A1 (Chapple) also disclosed a variable displacement motor driven by a fixed displacement pump, but with the motor displacement controlled according to the measured wind speed.
However, the input power to renewable energy devices is unpredictable from second to second, due to gusts and turbulence. This creates undesirable variations in the power output to the electricity network. For this reason it has been proposed to use a fluid store connected between the pump and motor, and to control the pump and/or motor to achieve the second-by-second storage and retrieval of excess energy in the fluid store.
It is known to provide a WTG comprising an electronically-commutated pump (i.e. one in which individual working chambers could be deactivated to vary the displacement of fluid by the pump each revolution, and thus the torque applied to the rotor). The rotor torque can be controlled to maintain a desired ratio of rotor speed to measured wind speed, while the turbine (functionally equivalent to the motor) or motor respectively can be controlled to maintain constant pressure in the fluid store (typically a pressure vessel or a flywheel or a vacuum store). U.S. Pat. No. 4,496,847 disclosed in addition constricting valves for, when necessary, isolating the pump from the turbine, thereby raising pressure to control the device against rotor overspeed. U.S. Pat. No. 4,280,061 (Lawson-Tancred) disclosed another WTG whereby the pump displacement was controlled according to the square of rotor speed and the motor was controlled to maintain constant pressure, in which the fluid store was a weighted hydraulic ram. U.S. Pat. No. 4,274,010 (Lawson-Tancred) disclosed in addition the ability to turn the generators/motors on and off intermittently according to power availability.
It is also known to provide a WTG with an electronically-commutated pump and a hydraulic accumulator as the energy store. While accumulators are a cheap and reliable energy store, their fluid pressure must vary very widely in operation. It is therefore not possible to apply the control strategies of other prior art (which control the pressure to set the torque on the rotor, or control the motors to maintain a predetermined pressure) to a WTG comprising accumulator energy storage, while maintaining the optimum tip speed ratio in order to extract the optimum amount of energy in all conditions.
An additional problem of hydrostatic WTGs of the prior art is that the efficiency of fluid working machines changes with operating pressure, and therefore energy may be unnecessarily lost when the control method dictates working at other than close to the optimum operating pressure.
The present invention aims to address one or more of the above problems and to provide a control method for a renewable energy device comprising a hydrostatic transmission and a cheap and reliable energy storage device, which is able to extract energy efficiently in all conditions, while providing a less variable energy output than would naturally occur.